The complaint is describes as “stacked” or “early” shifts; where the transmission will shift to 4th gear in less than 100 feet from takeoff. In this particular case it’s a 2000 Blazer 4X4 with a 4.3L engine. These vehicles are equipped with a New Venture transfer case; either model 233 or 243. Several people responded to this post, all pointing toward a 4WD Lo feature.
Here’s what happens: When you select 4WD LO the computer recalculates the speedometer reading (vehicle speed) it uses to determine shift timing. While in 4WD LO the transmission will upshift very early (e.g. 1-2 @ 5mph, 2-3 @ 10mph, 3-4 @ 15mph). This is based on the low range of the transfer case and keeps the engine RPMs normal throughout the shifts. The transfer case uses a switch to signal the computer when the 4WD LO range is selected. The switch grounds a wire to the PCM which initiates the secondary shift strategy.
If the 4WD LO switch wire shorts to ground the PCM will use the secondary shift strategy, resulting in a stacked-shift complaint. This problem is usually the result of the wire in the harness breaking and shorting to ground or to another wire rather than the switch itself. If the 4WD LO wire shorts to another sensor or signal wire the complaint can be intermittent or only under certain conditions. For example, there have been instances where the 4WD LO wire shorts to the TCC command wire, in which case the secondary shift strategy goes into effect when the PCM commands the converter clutch on.
TESTING: You can test for this problem very easily. Connect a scan tool and monitor the 4WD LO switch circuit. If it says “ON” or “YES” (model dependent) then you know the switch wire is grounded. You can also check the vehicle speed on the scan tool. If the secondary shift strategy is activated you’ll see the MPH about double that of the actual MPH.
IMPORTANT: All GM trucks use this feature — even 2WD vehicles. Even though the 2WD does not have a 4WD LO switch they still have the wire in the harness and the PCM will react the same as a 4WD version.
THE FIX: On 4WD vehicles you must locate the source of the short or run a new wire so the circuit operates normally. On 2WD versions you can, as an option, disconnect the 4WD LO wire from the PCM. However, this complaint is often the result of damage to the wire harness and other circuits may be affected by the damage so it’s best to locate and repair the harness.